The present invention relates to processes for separating the noble fission gases xenon and krypton from waste gases from nuclear plants, particularly from the dissolver exhaust gases of a reprocessing plant for irradiated nuclear fuel and/or breeder materials. The waste gas which is treated in the present invention is a prepurified waste gas which has been substantially or practically completely freed of its major contaminating components, namely aerosols, NO.sub.X, CO.sub.2, water vapor, iodine and RuO.sub.4. The prepurified waste gas thus contains essentially only Xe, Kr, N.sub.2 O, O.sub.2, N.sub.2 as well as small quantities of CO.sub.2.
Treatment of prepurified waste gas can be accomplished by bringing the prepurified gas into contact, in countercurrent, with liquid difluoro dichloro methane (Cl.sub.2 CF.sub.2) as an absorption agent, whereby Xe, Kr, N.sub.2 O and CO.sub.2 are absorbed and small quantities of O.sub.2 and N.sub.2 are absorbed and are thus removed from the waste gas. The so charged liquid absorption agent is then heated to the boiling temperature of Cl.sub.2 CF.sub.2 in order to evaporate part of the liquid absorption agent and to desorb an absorbed noble gas. The desorbed noble fission gas is then separated from the absorption agent vapor by condensation of this vapor. The Cl.sub.2 CF.sub.2, which has now been freed from the absorbed noble fission gas and condensed, is then circulated and reused.
The present invention further relates to a column arrangement for implementing a process for separating the noble fission gases xenon and krypton from the waste gases of nuclear plants. This column arrangement comprises three column sections--upper, intermediate, and lower--which are each provided, in their mass transfer zones, with means for mass transfer. The arrangement further includes an absorption agent evaporator, a cooling device connected upstream of the column arrangement, and means for circulating the absorption agent.
Cryogenic, adsorptive, and absorptive processes have been, and are currently being developed for separating noble fission gases from the dissolver exhaust gas in reprocessing plants. Cryogenic processes have a number of disadvantages, including the fact that they operate under pressure, accumulate large quantities of fission kryptons and require complicated and expensive preliminary purification of exhaust gases. This reduces safety and operability of the cryogenic systems. The adsorptive processes also contain disadvantages, including the fact that they operate discontinuously and require extremely frequent actuation of valves which are subject to malfunction.
The development of an absorption process for the separation of noble fission gases on the plant scale is taking place only in Oak Ridge, Tenn. The process employs Cl.sub.2 CF.sub.2, also known as "R-12" or "Refrigerant 12, " as the absorption agent and is disclosed in detail in German Offenlegungsschrift No. 2,831,564 which corresponds to U.S. Pat. No. 4,129,425 to Stephenson et al.
This process separates Xe and Kr together. Since the quantity of fission xenons is approximately ten times the quantity of fission kryptons, the Xe must be separated from the Kr in a further process step in order to realize an economically small, final storage volume and to commercially utilize the already inactive fission xenon.
An operating pressure up to 30 bar is used for the process.
In this process, larger quantities of contaminants are brought in. These are removed from the R-12 absorption agent by a subsequent distillation.
Due to the high operating pressure, low operating temperatures, down to about-80.degree. C. are selected.
The Kr is accumulated at a certain location in the column and is there removed continuously or discontinuously. When this process is used for the separation of the fission krypton from the dissolver exhaust gas of reprocessing systems, it has several disadvantages.
First, operation under pressure, particularly in the nuclear area, constitutes a high safety risk since a leakage may release the accumulated radioactive inventory. Expensive additional measures are therefore required to limit the safety risk. Moreover, an exhaust gas compressor is needed with this process. Regarding the introduction of contaminants, it is much more complicated to remove the contaminant from the absorption agent once introduced, for example, by means of process integrated distillation, than to freeze them out beforehand.
Although the higher than necessary operating temperature of the noble gas washers in this prior art process facilitates cooling, this is greatly overcompensated by the resulting reduction of separation selectivity, increased danger of corrosion and the costs for recovery of the evaporated R-12 from the purified waste gas. Moreover, the increased absorber operating temperature increases the circulating flow and thus the energy requirements during separation. The location in the column where the Kr accumulates and is removed depends on the operating conditions, as for example the waste gas quantity, and must be maintained at the point of discharge under fluctuating operating parameters by means of additional measuring, control and regulating devices which are subject to malfunction.
With the discontinuous discharge of the process, a somewhat higher Kr inventory accumulates between discharge periods than with a continuous discharge. Although such accumulation is low, it increases the radiolytic decomposition of the R-12, which is proportional to the Kr inventory, and which produces corrosive products, and it also increase the quantity of radioactivity that might be released in case of malfunction.
Finally, to separate the Xe from the Kr, a process has been developed which freezes out the Xe in cooling traps. This freezing process has the disadvantage of being discontinuous, and there is high contamination of the frozen Xe with radioactive fission krypton so that further purification steps are required which must be implemented under complicating and cost inefficient radiation protection measures.